1. Field of the Invention
This invention relates to a portable semiconductor memory unit, such as an IC card used as the external memory device of a personal computer and so on, and more particularly, it relates to a structure of a portable semiconductor memory unit devised to exclude the influence of electrostatic discharge.
2. Description of the Prior Art
In FIGS. 1 and 2, there is shown a conventional IC card of the type having conductive panels on its surface.
In those drawings, a card body 1 has conductive panels 2 and 2', which are made of conductive metal, for example, on its surface. A connector 3 having a plurality of connecting terminals therein, is provided at the side of the card body 1.
A circuit grounding terminal 4 is provided in the connector 3, and is connected to a grounded circuit or ground interconnection 5a of a circuit board 5. The circuit board 5 is provided in the card body 1, and electronic elements 6, such as an integrated circuit module, are arranged thereon.
When the card body 1 is loaded into the read/write device of a personal computer, the circuit grounding terminal 4 comes in contact to a grounded circuit of such device, and is grounded hereby.
An internal battery cell 7 is supported by the battery contacting members 8 and 9, the ends of which are fixed to the circuit board 5 and respectively connected to printed circuits provided on the circuit board 5.
The battery contacting member 8 is contacted with the negative electrode of the battery cell 7 and directly connected to the circuit grounding terminal 4 through the grounded circuit 5a of the circuit board 5.
The battery contacting member 9 is contacted with the positive electrode of the battery cell 7. Insulating layers 10 and 10' are respectively provided on the inside of the conductive panels 2 and 2' to prevent the electric elements 6 and the battery contacting members 8 and 9 from short-circuiting each other. The insulating layers 10 and 10' are formed by coating insulating material, such as synthetic resin, on the panels 2 and 2'.
The conductive panels 2 and 2' are connected to each other by a conductive coiled spring 11 inserted between them, and those panels provide the internal circuits in the card body 1 with an electromagnetic shield effectiveness.
As mentioned above, in the conventional IC card with the conductive panels on its surface, these conductive panels 2 and 2' are electrically insulated from the circuit grounding terminal 4.
When the IC card is inserted into the read/write device of the personal computer, the circuit grounding terminal 4 is grounded through the grounded circuit of the device. If low voltage static discharge noise (ESD noise) which is under the electrostatic breakdown voltage between the conductive panels 2, 2' and the circuit grounding terminal 4 is applied to the panels 2 and 2', the noise is cut off by the conductive panels 2 and 2'.
As a result, the internal circuits are not influenced.
If the ESD noise is at high voltage which is over the electrostatic breakdown voltage between the conductive panels 2, 2' and the circuit grounding terminal 4, the noise discharges through the conductive panels 2, 2', the internal circuits and the circuit grounding terminal 4.
Since there is no specific discharge channel within the IC card and such a channel is very unstable, the discharge current often flows into the internal electric elements 6, resulting in the destruction of the electric elements 6, changes of data therein or other material troubles.